The present invention relates, in general, to nondestructive examination (NDE) inspection techniques and, in particular, to a method and apparatus which uses artificially generated, broadband acoustic sound waves to interrogate an insulated piping structure and detect surface corrosion under the insulation on the piping structure.
There are enormous numbers of insulated piping systems utilized in industry, especially in the chemical and petrochemical arena. Many of these systems exist in corrosive environments where corrosion can occur unnoticed under the thermal insulation. A nondestructive examination technique for detecting surface corrosion on a piping structure is known where the inspection is performed using low frequency eddy current techniques. As described in U.S. Pat. No. 5,351,655 to Flora et al., assigned to The Babcock & Wilcox Company, assignee of the present invention, this technique does not require prior insulation removal along that portion of the piping structure being inspected. While the technique is very effective, it does require complete scanning of every pipe to ensure confidence of adequate inspection. A relatively labor-intensive effort is therefore required for thorough inspections; even to deduce the absence of corrosion. The effectiveness of the eddy current technique is also sensitive to variations in lagging types, connections, insulation moisture, etc.
Various NDE techniques are known to examine structures and detect the presence of flaws, leaks or corrosion in such structures, including what are referred to as acoustic emission (AE) techniques. The following list of patents and technical references are representative of various AE equipment and techniques known in the art.
Robertson et al. (U.S. Pat. No. 5,134,876) discloses an acoustic emission leak simulator used for non-destructive simulation of a leak in a structure.
Coulter et al. (U.S. Pat. No. 4,858,462) discloses a method and apparatus for locating a leak which produces a continuous acoustic emission having background noise and spikes. Spaced apart detectors produce signals having background noise and spikes corresponding to those of the continuous acoustic emissions and by measuring the offset between the spikes the relative position of the acoustic emission, and the relative position of the leak, can thus be determined with respect to the position of the detectors. A floating detection threshold is used and applied to the signals for eliminating background noise.
Askwith et al. (U.S. Pat. No. 5,031,456) discloses a method of detecting a void-related fault in a metal or metal alloy component. The invention is particularly useful in the detection of microbiologically-induced corrosion (MIC). Inter alia, the invention of Askwith et al. comprises filling a void-related damage with a fluid (or maintaining the component in conditions so that the void-related damage is filled with the fluid); thermally treating the component and fluid so as to produce a detectable acoustic signal; evaluating parameters conditioned to the signal; and correlating the parameters and conditions with the severity of the damage. The fluid within the MIC produced cavity or pits is thermally stimulated causing it to flow and/or boil. The acoustic energy detected during the thermal stimulation leads to the detection, location, and severity of the MIC damage.
Sugg et al. (U.S. Pat. No. 4,738,137) discloses acoustic emission non-destructive testing, in which broadband frequency noise is distinguished from narrowband acoustic emission signals, the latter being valid events indicative of structural flaws in the material being examined. This is accomplished by separating out those signals which contain frequency components both within and beyond (either above or below) the range of valid acoustic emission events.
Blackbum (U.S. Pat. No. 4,732,045) discloses a method for the rapid acoustic emission testing of pressure vessels wherein the flow rate of the pressurizing fluid is increased while maintaining the velocity of the pressurizing fluid, at an entrance of pressure vessel, below the velocity which would cause the generation of flow noise. An acoustic sensor on the pressure vessel being examined provides signals indicative of the acoustic emissions produced by the vessel during pressurization.
Rettig et al. (U.S. Pat. No. 3,946,600) discloses a method for detecting and monitoring corrosion by sensing the spontaneous propagation of elastic waves produced in materials as a result of corrosion phenomena. Certain corrosion reactions generate elastic waves which may be detected by sufficiently sensitive instrumentation to provide an identifiable acoustic signature, and correlation of these signatures with known standards provides a non-destructive inspection technique for monitoring invisible or hidden corrosion. The acoustic emission signals that are generated by corrosion reactions are characterized by a series of pulses (noise bursts) with extremely short rise times. The patent also contemplates the inclusion of controlled environments both in establishing the standard emission samples, and in making field tests, which include thermal heating via a heat lamp or electrical resistance heating to supply a controlled amount of thermal energy to the corrosion-suspect joint. The thermal energy accelerates common corrosion reactions to simplify or hasten the acquisition of the output data. Other aggressive environments may be employed to enhance or accelerate the acoustic emission process of corrosion, such as high humidity.
Leschek (U.S. Pat. No. 4,039,767) discloses a method of calibrating acoustic emission transducers, wherein random acoustic white noise is transmitted into an acoustic medium to establish a multi-mode reverberant sound field. Output responses from a standard transducer and the acoustic emission transducer to the reverberant sound field are obtained and compared to determine the sensitivity of the acoustic emission transducer.
Romrell (U.S. Pat. No. 4,033,179) discloses methods and apparatus for identifying the source location of acoustic emissions generated within an acoustic conductive medium, and which is particularly applicable to on-line monitoring of welding process to identify and locate flaws.
Hein, Jr. et at. (U.S. Pat. No. 4,297,885) discloses a method for detecting initiation and propagation of cracks in materials by adhering an acoustical emitter in the area of the material to be monitored, and then monitoring for acoustic signals emitted by the emitter.
Latimer (U.S. Pat. Nos. 4,685,334 and 5,243,862) disclose methods for ultrasonic detection and confirmation of hydrogen damage in boiler tubes. Hydrogen damage occurs on the inner surface of the boiler tubes. In the '334 patent, ultrasonic shear waves are introduced via a pitch-catch technique in the axial and circumferential directions of a boiler tube to detect the presence of hydrogen damage therein. Latimer '862 discloses a further enhancement of a hydrogen damage inspection technique which confirms that damage detected using refracted shear waves is indeed hydrogen damage, and not other inside diameter surface conditions.
Birring et al. (U.S. Pat. No. 4,890,496) discloses a method for detecting hydrogen attack by ultrasound wave velocity measurements in which ultrasound waves are transmitted on one transmitter to another along a fixed path through a steel body such as pipe. The velocity of the ultrasound wave is determined with an accuracy of one-tenth of a percent, and a decrease of a velocity by more than two percent indicates hydrogen attack. In one embodiment, either refracted longitudinal or refracted shear waves are transmitted, and the velocities thereof are determined. In another embodiment, a creeping wave is transmitted from one transducer to the other transducer with the creeping wave including a surface wave and a subsurface wave. In a third embodiment, the ultrasonic waves are transmitted into the material and the backscattered ultrasound is measured. Increases in the backscattered ultrasound are related to hydrogen attack.
Alers et al. (U.S. Pat. No. 4,289,030) discloses a nondestructive test device for detecting a flaw proximate to a welded seam in a pipe, in which a horizontally polarized shear wave is generated in a wall, and the pipe is monitored to detect a reflected horizontally polarized shear wave. Times of arrival of the generated and reflected waves are correlated to determine the circumferential position of the flaw.
Nuspl (U.S. Pat. No. 5,351,655) discloses an acoustic signal collection manifold used to monitor leaks in furnace tubes.
The Nondestructive Testing Handbook-Second Edition, Vol. 7 (Binks, Green Jr., and McIntire, Eds., Copyright .COPYRGT.1991 American Society for Nondestructive Testing, Inc.), "Ultrasonic Testing"; Section 12: Material Property Characterization - Part 2, "Material Characterization Methods", pp 386-387, discloses various material characterization methods where acoustic emission has been used to identify crucial material variables. As discussed at page 387, acoustic emission frequencies can range from the audible (sonic) to several megahertz (ultrasonic). Traditional applications of acoustic emission are disclosed, wherein passive sensors are fixed to the surface of the test object and are selected to assure sensitivity to signals generated at some distance by microdisturbances and other weak sources. The operational methods include event counts, ringdown counts, energy or amplitude distribution analysis and frequency spectrum analysis. The primary objective of traditional acoustic emission testing is the detection and location of incipient discontinuities. It can be used to monitor the presence and severity of growing cracks, plastic deformation or delaminations. It can also be used to monitor structural integrity and dynamic response and for inferring the current internal condition or state of degradation in structural components. Examples of in-process monitoring include solidification processes such as spot welding and heavy section welding.
Page 387 of this reference also includes the following paragraph:
"Another objective of acoustic emission testing is source characterization. This is hampered by signal modifications in transducers, instrumentation and especially the material. Signal modification by material microstructure, texture, diffuse discontinuity populations, mode conversions and reflections at boundary surfaces make it inherently difficult to quantitatively infer the exact nature of emitting sources. Because source characteristics are usually unknown, acoustic emission is not used for quantitative characterization of microstructure or material properties."
Nondestructive Testing Handbook-Second Edition, Vol. 5 (Miller and McIntire, Eds., Copyright .COPYRGT.1987 American Society for Nondestructive Testing, Inc.), "Acoustic Emission Testing"; Section 2: Macroscopic Origins of Acoustic Emission - Part 5, "Corrosion and Stress Corrosion Cracking", pp 58-61; Section 4: Wave Propagation - Part 4, "Attenuation of Waves", pp. 103-107 discloses that acoustic emission techniques have been used for monitoring and detecting the initiation of propagation of cracks resulting from different forms of corrosion. Further, hydrogen embrittlement itself has also been monitored with acoustic emission techniques and acoustic emission rates obtained at a value close to that of the critical stress intensity factor can be used to predict the onset of unstable crack growth. Section 4, Part 4 discloses various factors affecting the attenuation of waves, the term attenuation being used in a general sense to mean the decrease in amplitude that occurs as a wave travels through a medium. Various mechanisms have been identified that cause attenuation, and include geometric factors, dispersion effects, energy loss mechanisms, and attenuation caused by scattering and diffraction due to the media having complex boundaries and discontinuities (such as holes, slots, cavities, cracks and inclusions). Attenuation tests on actual structures have been performed which indicate that attenuation is a function of frequency (see page 106, FIGS. 14 and 15). Additionally, FIG. 17 at page 107 confirms that surface coatings applied to a structure can affect the attenuation. FIG. 17 shows the attenuation effects of foam insulation panels applied to an aluminum panel, that the attenuation that occurs is very sensitive to the technique used to apply the foam; i.e., whether it is insulation that is merely sprayed on or glued on to the surface.
A nondestructive evaluation technique to detect the presence of such corrosion without the cost inhibited task of insulation removal and replacement would be welcomed by the industry.